1. Field of the Invention
The present invention relates to a sintered ceramic composite body with improved toughness and mechanical strength and a method of manufacturing such a sintered ceramic composite body.
2. Prior Art
Ceramic materials have long been used as refractory materials and chemical materials since they are highly resistant to corrosion and heat and high in hardness. The recent advance in the chemical technology allows highly pure materials to be refined and synthesized, and the process control technology makes it possible to produce ceramic materials having widely varied properties, which have attracted much attention in the art. Heat-resistant alloys have heretofore been employed in applications at high temperatures or in adverse environments, such as gas turbine blades. However, there has been a demand for more excellent high-temperature structural materials in view of the recent trend in the market for higher performances. Ceramic is recognized as an important material which meets the requirements in such uses, because it is much better than other materials with respect to heat resistance, acid resistance, and corrosion resistance.
Ceramic materials such as silicon nitride, alumina, silicon carbide are generally brittle, and many of these materials have a fracture toughness of 5 MNm.sup.-3/2 or less. Various attempts have heretofore been made to improve the toughness of ceramic materials.
One effort to toughen a ceramic material involves the addition of needle-like components such as whiskers, fibers, or the like as a reinforcement material, as disclosed in Japanese Laid-Open Patent Publication No. 59-30770. It is considered that the toughness of a ceramic material with such a reinforcement material is increased by the crack deflection effect in which cracks produced in the ceramic by whiskers or the like dispersed therein are bent, or the whisker pullout effect.
However, it is difficult to disperse needle-like reinforcing elements uniformly in a ceramic material. If fibers are used as a reinforcement material, the dispersed fibers tend to be entangled together into a fiber mass in the ceramic.
Japanese Patent Publication No. 59-25748 discloses a method of toughening an alumina ceramic material with zirconia added as a reinforcement material. According to the disclosed method, zirconia is left as a metastable tetragonal system in alumina down to room temperature, and the mechanical properties of the ceramic material at room temperature are greatly improved by residual compressive stresses which are produced due to a volume expansion by 4% upon crystal system transformation that is caused from the tetragonal system into a monoclinic system owing to stresses at tip ends of produced cracks.
Even with the above ceramic toughening method, however, if the ceramic material is left for a long time in atmosphere at a temperature higher than about 900.degree. C. which is the transformation temperature, then zirconium oxide and the base material which is a nonoxide react with each other to the extent that the properties of the base material can no longer be maintained. As a result, the above toughening effect is not achieved.
Japanese Laid-Open Patent Publication No. 61-174165 shows dispersing silicon carbide in an alumina material for an increased mechanical strength. According to the disclosed method, particles of silicon carbide having an average diameter of 3 .mu.m or less or whiskers of silicon carbide having a diameter of 1 .mu.m or less and a length of 20 .mu.m or less are independently dispersed in an alumina material, applying localized residual stresses alumina grain boundaries, so that the mechanical properties of the composite material at high temperature are improved.
Japanese Laid-Open Patent Publication No. 62-246865 discloses a sintered body of silicon nitride with a rare earth, MgO, and ZrO.sub.2 added as sintering aids. Japanese Laid-Open Patent Publication No. 63-100067 shows a sintered material of silicon nitride which contains two or more of Y.sub.2 O.sub.3, Er.sub.2 O.sub.3, Tm.sub.2 O.sub.3, Yb.sub.2 O.sub.3, and Lu.sub.2 O.sub.3. However, silicon carbide is not contained as a reinforcement material.
In efforts to increase the toughness of a ceramic material with dispersed whiskers, fibers, or the like, it is difficult to disperse whiskers, fibers, or the like uniformly. Even if these reinforcement elements can be dispersed relatively uniformly, it is impossible to produce sintered ceramic composite bodies of good properties unless specially processed in the manufacturing process. Use of whiskers, fibers, or other reinforcement materials is highly expensive. In the case where particles of zirconium oxide are dispersed for increased toughness, no toughening effect is achieved at high temperatures where no crystal system transformation progresses. If the ceramic material is left at a high temperature for a long period of time, then zirconium oxide and the base material which is a nonoxide react with each other, and the properties of the base material are no longer maintained.